The spread of mobile telephones has enabled visitors to announce their arrival at a destination by making a call from their own mobile telephone to a mobile telephone or fixed telephone belonging to a resident being visited, without requiring an intercom.
When a mobile telephone in place of an intercom, the resident cannot easily determine whether or not the visitor is actually present at their home. Also, a visitor who does not know the telephone number of the resident is unable to announce their visit.
As such, when a mobile telephone is used instead of an intercom, a system is sought that enables determination of whether the visitor has actually arrived, and enabling any visitor to announce their visit without requiring the resident to publicise their telephone number.
Patent Literature 1 discloses an intercom system using a mobile telephone terminal. This system includes a mobile telephone terminal owned by a visitor, an entrance device installed at an entrance, an indoor device installed indoors, and a mobile telephone terminal owned by a resident.
The visitor uses their mobile phone terminal to connect to the entrance device. The entrance device then reads the telephone number of the visitor's mobile telephone device and notifies the indoor device of the telephone number so read. The indoor device verifies the visitor by cross-referencing the received telephone number with a telephone number database. When the verification is successful, the indoor device wirelessly connects the visitor's mobile telephone terminal, which is connected to the entrance device, with the mobile telephone terminal belonging to the resident. This enables communication between terminals.